This relates generally to electronic devices, and more particularly, to electronic devices with displays.
Electronic devices often include displays. For example, cellular telephones and portable computers may have displays for presenting information to a user.
Liquid crystal displays contain a layer of liquid crystal material. Display pixels in a liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the display pixel.
Substrate layers such as color filter layers and thin-film transistor layers are used in liquid crystal displays. A thin-film transistor layer contains an array of the thin-film transistors that are used in controlling electric fields in the liquid crystal layer. A color filter layer contains an array of color filter elements such as red, blue, and green elements. The color filter layer provides the display with the ability to display color images.
In an assembled display, the layer of liquid crystal material is sandwiched between the thin-film transistor layer and the color filter layer. The center of the display forms an active area that is occupied by an array of pixels. The border of the display is inactive and may contain support circuitry. In the inactive border, opaque masking material is used to prevent stray light from escaping from the display and to hide support circuitry from view by a user of the display.
The opaque masking material is formed from an opaque material such black ink. The black ink is formed from a photoimageable polymer that contains a black filler material. The black ink is an insulator, but is generally not able to withstand high voltages. During electrostatic discharge events in which a user touches the edge of the display, high voltages such as voltages on the order of 10 kV or higher may be applied to the black ink. The black ink cannot reliably withstand these high voltages, so static charge may migrate into the active area of the display through the black ink. This disrupts the electric field distribution within the liquid crystal material of the display and leads to visible artifacts. As an example, the pixels of the display near the border may exhibit a visible color cast, because pixels of different colors respond differently to the disruption from the static charge.
In an effort to enhance immunity to electrostatic discharge, some displays have opaque masking layers that are recessed from the outermost edge of the display. This creates a high resistance air gap that can resists electrostatic discharge, but involves the addition of an overlapping opaque gasket structure on the outside of the layers in the display to prevent light leakage. The overlapping opaque gasket structure may undesirably increase the bulk of the display structures at the edge of the display.
It would therefore be desirable to be able to provide improved electronic device displays with structures that prevent display damage from electrostatic discharge.